Two inputs on the VCAs are nice

[EricK]

The two inputs on the 902 VCA are nice.

I have a sequence pitch voltage going to my 921a. My 2 921B oscs are going to the CP3 mixer. One of the outputs of the mixer is going to the 904a, and from there it is going to the 902.

The other output of the mixer is going to the FFB. The output there is going to the Bass Murf, which is running to the second input of the VCA.

The VCA is going to the 104z which is going to the board. So you have a sequence getting that cliché dotted 8th delay but you also get certain frequencies of the FFB accented and delayed at different times.



If you are just droning, the two oscillators will have some nice phase cancellation as usual. But you get interesting effects when you route the negative voltages of those oscs to the FFB and then into the VCA. Those "four" oscs generate some interesting timbres.


I'm doing all of this on a Moog clone modular but the approach is the same. Sound files to come later.

[analogmonster]

While cloning the 902 I've seen that the input buffer works as a difference amplifier with one input connected to each side so the two inputs are processed with a 180 degree phase difference.

[EricK]

That's interesting because I'll send the positive and negative outputs from the mixer into the VCA.

[MC]

The audio signal chain is differential, but they don't have to be 180 out of phase.

This VCA offers a few tricks because of the differential inputs.

You can get a highpass filter mode by mixing direct signal and output from a lowpass filter. It can be used as a ring modulator but the common mode rejection won't be as good as a dedicated ring modulator.

Just peeked at the schematic - interesting that a predistortion diode bridge is used in the output stage to improve the fidelity.

[MC]

That's interesting because I'll send the positive and negative outputs from the mixer into the VCA.

You're basically running balanced lines. This patching is 180 out of phase which will double the volume by 6dB in the VCA, and cancel induced common mode noise.

[EricK]

MC,
See, this is where it is getting beyond my scope.

Are my signals really out of phase? From what Analogmonster wrote, it isn't clear. I run a negative and a positive from my mixer to the VCA.
From what I understood of what he wrote, Positive + Positive = 180 out of phase?


So do I get positive + Negative = in phase?
Positive + Negative = Negative plus positive (no real change)?

I did notice a significant change to the sound, but I never threw it on the scope to see how it looked.


Eric

[analogmonster]

Hey, you can talk to me directly

If you feed the same signal to the differential inputs of the 902, the result is zero, because one side is inverted. This is the same as if you would ADD a signal to your original which is 180 degrees out of phase (meaning just the AC components of the signal, any DC offset ist just inverted). If you do a simple mix of different signals (e. g. with a CP3A), all signals are added in phase. Mixing the same stuff with the differential inputs of a 902 means whatever you add on one side of the 902 is added with 180 degrees out of phase to the sum, compared with the input of the other side of the 902. That's why you get a different output sound spectrum if you compare a signal mix via CP3A with a signal mix via 902.

To increase confusion, the OUPUTS of the 902 are also differential, providing output sum signals which are 180 degrees out of phase compared to each other Be aware of that for all post processing.

[EricK]

AM,
Es tut mir lied!
I'm sorry I wasn't trying to be rude.

What I normally do is take the positive out from the CP > VCF > VCA. Then I will take the negative output > FFB > VCA. So going into the VCA they are already out of phase.

So you are saying that this reverses an already out of phase signal into an out of phase signal?


(See, I just have no clue about this kind of stuff.)

Thanks,
Eric

[MC]

Not responsible for re-living nightmares of math classes...

Out of phase signals are opposite polarities, IE one is positive the other negative or vice versa.

So assume one signal is plus one volt and the other is minus one volt, and you route them to a differential input. A differential input subtracts the input signals.

Thus positive one minus negative one - subtracting a negative number changes it to addition yielding one PLUS one - equals an output of two. This doubles the output signal which is a 6dB increase.

[analogmonster]

AM,
Es tut mir lied!
I'm sorry I wasn't trying to be rude.

No problem

What I normally do is take the positive out from the CP > VCF > VCA. Then I will take the negative output > FFB > VCA. So going into the VCA they are already out of phase.

So you are saying that this reverses an already out of phase signal into an out of phase signal?


(See, I just have no clue about this kind of stuff.)

Thanks,
Eric

Yes, as MC said, if you feed an inverted signal into an inverting input of a differential amp you do an addition, not a subtraction.

Or, to be the smartass of the day:

1 - ( +1) = 0
1 - ( -1) = 2

[EricK]

(I just noticed the spelling on my German was off.)

Hey thanks for simplifying the explanations, guys.